1. Field of the Invention
The present invention relates to a golf club, and more particularly, it relates to a golf club comprising a golf club head, having a hardly breakable face, hardly reducing the carry of a golf ball also when making an off-centered shot.
2. Description of the Prior Art
Japanese Patent Laying-Open No. 9-168613 (1997) describes a golf club head according to first prior art. This gazette discloses a golf club head of a hollow structure provided with a hitting portion having sufficient strength for withstanding impact located at the center of a face and a portion having a small spring constant located around the same.
Japanese Patent Laying-Open No. 9-192273 (1997) discloses a golf club head of a metal according to second prior art, which is provided with a face center part in a thickness having sufficient strength for withstanding impact applied by collision with a golf ball and a peripheral part having a smaller thickness than the face center part.
Japanese Patent Laying-Open No. 9-299519 (1997) discloses a wood golf club head according to third prior art, which is provided with an annular groove on the inner surface of a face wall part to enclose the central portion of the inner surface.
An important factor required to a golf club is the ability of increasing the carry of a golf ball. When the carry is remarkably increased, the player can readily make the next shot to gain a good score. The carry remarkably depends on the position of the golf club hitting the golf ball. Dissimilarly to a professional golf player or a skilled nonprofessional player, a general player hits the golf ball at various portions such as upper, lower, right and left portions of the face of the golf club head. Therefore, while the golf ball carries enough when colliding with a sweet spot (SS) of the golf club head, the carry is extremely reduced when the golf ball collides with another portion of the face out of the sweet spot.
Bounce of the face of the golf club head is a factor remarkably concerned in the carry of the golf ball.
In order to improve the bounce of the golf club head, rigidity of the face must be reduced, i.e., the face must have a large quantity of vertical flexure. This point is now described.
FIG. 20 illustrates the relation between restitution coefficients and spring constants of golf club heads. Some wood golf club heads were selected for colliding golf balls with sweet spots (SS) of the golf club heads and measuring speeds of the golf balls before and after the collision, in order to obtain the restitution coefficient of each golf club head through the following numerical formula (1):Vout/Vin=(eM−m)/(M+m)  (1)where Vout and Vin represent the speeds of the golf ball after and before the collision respectively, M represents the weight of the golf club head, m represents the weight of the golf ball and e represents the restitution coefficient.
The spring constant of each golf club head was obtained by applying a vertical load (5 kN) to the sweet spot of the face and dividing the vertical load by the quantity of vertical flexure of the face.
It is understood from FIG. 20 that the spring constant and the restitution coefficient are extremely correlated with each other and the restitution coefficient is increased as the quantity of vertical flexure of the face is increased.
In order to increase the restitution coefficient, therefore, it is important to increase the quantity of vertical flexure of the face.
As described above, however, a general golf player hits the golf ball at various portions such as the upper, lower, right and left portions of the face of the golf club head. Therefore, it is insufficient to merely render the face center of the golf club head flexible but bounce in an offset shot at a position displaced from the sweet spot must be sufficiently increased.
In the first prior art (Japanese Patent Laying-Open No. 9-168613), the portion having a small spring constant is not arranged in response to the hitting point distribution of the player, and hence the carry of a golf ball is remarkably reduced by an offset shot although the ball carries enough when hit at the face center of this golf club head.
In the golf club head according to the first prior art provided with the portion having a smaller spring constant around the central hitting portion, further, metal materials having different spring constants must be connected with each other for forming the central portion and the peripheral portion of the face respectively with much labor at a high cost.
When the thickness of the portion around the hitting portion is reduced as compared with the hitting portion as in the prior art or an annular groove enclosing the hitting portion is formed on the inner surface of the face as in the third prior art, stress concentration is readily caused on the boundary between the portions having different thicknesses or the portion provided with the annular groove, to readily break the face by impact resulting from an offset shot.
In the golf club head according to the second prior art (Japanese Patent Laying-Open No. 9-192273), the peripheral portion is not arranged in response to the hitting point distribution of the player either and hence the carry of a golf ball is remarkably reduced by an offset shot although the ball carries enough when hit at the face center of this golf club head. Further, stress concentration is readily caused on the boundary between the portions having different thicknesses, to readily break the face by impact resulting from an offset shot.
In the golf club head according to the third prior art (Japanese Patent Laying-Open No. 9-299519), the carry of a golf ball is remarkably reduced by an offset shot similarly to the first prior art and the second prior art. Further, the annular groove and the central portion have remarkably different thicknesses, and hence stress concentration is readily caused on the boundary therebetween. Thus, the golf club head is readily cracked due to impact resulting from an offset or a flaw or a depression caused by a shot.